This invention relates generally to a feeder for transporting document pages seriatim across the exposure station of a reproduction apparatus, and more particularly to a vacuum document feeder which reduces the formation of undesirable artifacts on the copies of such document pages.
In order to efficiently utilize commercially available high-speed reproduction apparatus, such as electrophotographic copier/duplicators, document pages to be copied are automatically transported seriatim to and from the exposure station of the apparatus. One typical transport mechanism is a vacuum document feeder, such as that shown, for example, in U.S. Pat. No. 4,043,665, issued Aug. 23, 1977 in the name of Caldwell. Such a feeder employs an endless ported belt supported to traverse a path overlying the exposure station of the reproduction apparatus. A vacuum plenum associated with the belt tacks a document page to the belt for transport across the exposure station.
Some vacuum document feeders may cause undesirable artifacts to be produced in the copies of the document pages. These artifacts are caused by trapping of light during exposure of the document page at the exposure station. Light trapping frequently occurs when ports in the transport belt are superpositioned on the ports in the vacuum plenum. During exposure light passes through the belt and plenum ports when the document page does not fully cover the exposed belt area, or when the page is thin enough to transmit light (e.g., 16-pound paper). Light passing through the plenum ports is trapped within the plenum. The superimposed belt and plenum ports thus appear as dark spots, or shadows, in the reflected light image and are reproduced in the copies as undesirable artifacts.
Several means for reducing or eliminating the production of artifacts have been tried. For example, the transport belts may be formed of material such as porous foam or cloth, having vacuum apertures which do not extend directly from the front surface to the back surface of the belt. However, such materials typically do not have desirable wear characteristics, or are subject to clogging of the apertures, for example by paper and dust particles. Another means for reducing light trapping is shown in U.S. Pat. No. 4,047,812, issued Sept. 13, 1977, in the name of Hogan. The perforated transport belt in that patent has endless parallel imperforate bands. These bands lie in the areas corresponding to the top and bottom marginal edges of certain standard size document pages transported by the belt. Since the imperforate bands form a continuous reflective surface adjacent to such marginal edges, artifacts are reduced in the corresponding areas of the copies. However, artifacts may still be produced in the other areas of the copy (i.e., in the areas along the longitudinal opposed marginal edges, or in the case of thin document pages, within the body of the copy).